The present invention relates generally to techniques for automatically processing natural gamma radioactivity well logging data. More particularly, this invention relates to methods and apparatus for producing a representation of the clay content of formations surrounding a borehole from measurements of the total natural gamma radioactivity G, the content of the radioactive isotope potassium 40 (hereinafter called potassium or K), the content of the radioactive elements of the uranium family (hereinafter called uranium or U) and the content of the radioactive elements of the thorium family (hereinafter called thorium or T). T, U and K are responsible for nearly all gamma radiation encountered in the earth.
It is known that, since T, U and K are generally concentrated in clay, the natural gamma radioactivity of an earth formation is one of the most reliable clay indicators. The reliability of such indicator is, however, not as good as desired for the reason that several radioactive minerals are also encountered in sedimental areas which are not related to clay. This is the case in particular for the micaceous sands which contain potassium or for siderite that sometimes contains thorium. The presence of these radioactive minerals not related to clay is clearly indicated by the high value of the relative content (T/G or K/G ratio) of the radioactive element involved in the concerned layer compared to the values of the relative content of the same element in adjacent layers.
Another problem is due to the fact that a high value of the relative content of uranium (U/G ratio), i.e. a value significantly higher than those generally encountered in standard clay is often related, in sedimental areas, to the presence of organic materials. These materials have, throughout the course of the geological eras, favored the concentration of uranium in the formations. The marine shales which are rich in organic matters, notably phosphates and carbonates, have therefore an exceptionally high uranium content. It will be noted here that the expression "standard clay" covers clays whose relative TUK contents in the same sedimental bed are dispersed within a large, but relatively well known, range from which are excluded, for example, the relatively high uranium contents of the marine shales.
It is therefore a broad object of this invention to provide a technique for automatically processing natural gamma ray well logging data in order to determine a representation of the clay content of formations from the TUK contents when a disturbing radioactive element is present in some of these formations.